When h264_omx used, cannot start streaming client after server

Description

I am trying to stream a webcam to the network through the RDP protocol on a raspberry pi 2 B.

The problem is by doing so, I have to start the client (ffplay or vlc) first in order to receive images. If I start ffmpeg streaming first and THEN the client (ffplay or vlc), no images are displayed and ffplay outputs error messages:

On the client, to display the stream I first save the SDP file provided by ffmpeg on its output and do:

ffplay sdp.out

At the bottom of this description you will find a couple of investigations I made to prove the problem comes from the video encoder, but for the sake of compliance with the bug report rules, here is now the output of ffmpeg (ffmpeg -v 9 -loglevel 99 -i ...):

It didn't work so I tried on my desktop. Without the -force_key_frame option and by using libx264 codec, the result is the same as on the raspberry, ffplay cannot catch the feed in the middle of a stream (has to be started first). But when I use the option aforementioned, ffplay is able to catch on: